1. Field of the Invention
The present invention relates to a reciprocating compressor and is particularly suitable for a reciprocating compressor of small capacity and high pressure ratio that sucks combustible gas or toxic gas as working gas at low pressure and compresses the working gas at multiple stages and discharges the working gas at high pressure (for example, high pressure exceeding 70 MPa).
2. Description of the Related Art
For example, specifications covering the suction pressure and discharge pressure of a hydrogen compressor for a hydrogen supply station are determined by market needs and the supply pressure of a hydrogen supply source. There are cases where the pressure of hydrogen produced by a reforming plant or the like is as low as approximately several MPa to one-tenth of several MPa. On the other hand, there are market needs for requiring that the discharge pressure of a compressor exceeds 70 MPa so as to increase the amount of hydrogen fuel charged into a fuel-cell vehicle. Multiple compression stages more than two stages are required for such a specification of low suction pressure and high discharge pressure.
Among the conventional reciprocating compressors of multiple compression stages is a compressor disclosed in Japanese Patent Laid-Open No. H7 (1995)-189885. This reciprocating compressor is of the type in which three cylinders are fixed to the same crankcase and in which respective pistons reciprocating in these three cylinders are driven by the same crankshaft.
The crankcase of this compressor has the first to third cylinder fixing parts on the top surface and on the left and right sides. The crankshaft is rotatably supported in the crankcase and a connecting rod is coupled to the crank pin of the crankshaft. The first to third cylinder fixing parts are formed at positions shifted respectively by 90° in the rotational direction with respect to the rotational center of the crankshaft. The first to third cylinders are fixed to these cylinder fixing parts. The first to third pistons are slidably fitted in the respective cylinders. The tip of the connecting rod is coupled to the piston pin of each piston. Further, each cylinder has a suction valve and an exhaust valve that are opened or closed by the reciprocating motion of the piston pin. With this, when the crankshaft is rotated by a motor, the connecting rod is swung to reciprocate the piston.
Here, three compression parts, each of which is constructed of the cylinder, the piston, and the connecting rod, are arranged side by side in the axial direction. The second cylinder is used for high pressure and the first and third cylinders function as middle-pressure compressors and the compressed air pressurized to middle pressure by the reciprocating motion of the first and third pistons is compressed to high pressure by the second piston.
Further, among the conventional reciprocating compressors of multiple compression stages is a compressor disclosed in Japanese Patent Laid-Open No. 2000-192879. In this reciprocating compressor, a pair of opposed pistons are coupled to the first yoke and another pair of opposed pistons are coupled to the second yoke arranged in such a way that its direction is shifted by 90° with respect to the first yoke to construct four reciprocating compression parts. An electric motor part rotates a crankshaft to rotate a crank pin around the crankshaft to reciprocate the pair of pistons only in the direction of the first axis and to rotate another pair of pistons only in the direction of the second axis.
Here, the first yoke and the second yoke are arranged side by side in the axial direction. Further, four reciprocating compression parts are arranged at positions shifted respectively by 90° from the first reciprocating compression part and compress the working gas to high pressure in sequence.
In the reciprocating compressor disclosed in Japanese Patent Laid-Open No. H7(1995)-189885, three compression parts are arranged side by side in the axial direction. The constructing of a compressor of sucking air at low pressure and discharging the air at high pressure by use of the reciprocating compressor disclosed in this patent document results in increasing the size of the compressor in the axial direction and hence upsizing the compressor and further generating a large couple of forces in the crankshaft and by extension in its bearings, which raises the possibility of increasing loss and reducing the reliability of constituent parts.
Further, in the reciprocating compressor disclosed in Japanese Patent Laid-Open No. 2000-192879, the first yoke and the second yoke are arranged side by side in the axial direction. The constructing of a compressor of sucking air at low pressure and discharging the air at high pressure by use of the reciprocating compressor disclosed in this patent document results in increasing the size of the compressor in the axial direction and hence upsizing the compressor. Further, four reciprocating compression parts are arranged at positions shifted respectively by 90° from the first reciprocating compression part and compress the working gas to high pressure in sequence and the reciprocating compression part of the highest pressure is opposed to the reciprocating compression part the pressure of which is lower than this compression part by two stages. Hence, the pressure difference between these reciprocating compression parts applies a large load to the crankshaft to cause an increase in loss, which is not desirable.